Channel Estimation and Hybrid Architectures for RIS-Assisted Communications

Abstract

Reconfigurable intelligent surfaces (RISs) are considered as potential technologies for the upcoming sixthgeneration (6G) wireless communication system. Various benefits brought by deploying one or multiple RISs include increased spectrum and energy efficiency, enhanced connectivity, extended communication coverage, reduced complexity at transceivers, and even improved localization accuracy. However, to unleash their full potential, fundamentals related to RISs, ranging from physical-layer (PHY) modelling to RIS phase control, need to be addressed thoroughly. In this paper, we provide an overview of some timely research problems related to the RIS technology, i.e., PHY modelling (including also physics), channel estimation, potential RIS architectures, and RIS phase control (via both model-based and data-driven approaches), along with recent numerical results. We envision that more efforts will be devoted towards intelligent wireless environments, enabled by RISs.

RIS-assisted Wireless Channel Characteristic in Coal Mine Tunnel Based on 6G Mobile Communication System

“…on the right wall −2ma + (−1) m x 0 , on the left wall (13) In the same way, the coordinates of the mirror point reflected by m + n times on the upper and lower walls can be expressed as:…”

“…The challenges of channel estimation and the impact of feedback overhead in RIS-assisted systems are also under investigation. In the context of Orthogonal Frequency Division Multiplexing (OFDM) systems assisted by RIS, the authors in [12] and [13] introduced a transmission protocol that sequentially carries out channel estimation and reflection optimization. This protocol employs a low-complexity algorithm based on the strongest signal in the time domain to optimize the reflection coefficients after estimating the Channel State Information (CSI).…”

Spectral Efficiency Analysis of Hybrid Relay-Reflecting Intelligent Surface-Assisted Cell-Free Massive MIMO Systems

“…Another challenge in resource allocation of RISs is to acquire the channel state information (CSI) of reflected links. Most of the existing studies (e.g., [23], [26]) assumed the availability of perfect CSI, which is, however, much more difficult to acquire compared to the conventional systems without RISs [29], [30]. Concerning this challenge, the works [27] and [31] considered partial CSI, while efficient channel estimation schemes were proposed in [32]- [34].…”

Hybrid Active-Passive Reconfigurable Intelligent Surface-Assisted UAV Communications

“…A fully active RIS with all active elements can achieve better spectral efficiency [30]- [32], which, however, requires higher power consumption and hardware cost compared to the hybrid one. In [20], [21], [33], [34], active RIS elements with RF chains also enable processing of incident signals and channel estimation.…”